The Catalytic Partial Oxidation of N-Tetradecane on Rh and Sr Substituted Pyrochlores

by Haynes, Daniel

Abstract (Summary)

Catalyst deactivation by high levels of sulfur and aromatics limits the catalytic partial oxidation (CPOX) of diesel fuel into a H2-rich stream for fuel cells. These species poison traditional supported metal catalysts because they adsorb strongly to electron dense metal clusters and promote the formation of carbon on the surface. Therefore, it is logical to spatially distribute an active metal into the lattice of a chemically and thermally stable material to create an active catalyst surface that is less likely to accumulate carbon or be deactivated by sulfur. In this work, Rh metal only and Rh + Sr are substituted into lanthanum zirconate (LZ) pyrochlore (La2Zr2O7) to give La2RhyZr(2-y)O(7-?,) (LRZ) and La(2-x)SrxRhyZr(2-y)O(7-?) (LSRZ) catalysts. Their resistance to deactivation and carbon formation were examined by the CPOX of a mixture of model compounds chosen to represent diesel fuel. The results were compared to a commercial Rh/?-Al2O3 catalyst. Characterization results appear to confirm the Rh metal is distributed throughout the pyrochlore structure and is reducible. Activity screening with the CPOX of n-tetradecane (TD) with no other reactants shows that the Rh substituted in LRZ and LSRZ has activity comparable to the supported Rh/?-Al2O3, and each of these catalysts produces H2 and CO yields close to equilibrium levels. Effects of polynuclear aromatics (5 wt % 1-methylnaphthalene (MN) in TD), sulfur (1000 ppmw dibenzothiophene (DBT) in TD) and 5 wt % MN + 1000 ppmw DBT in TD on catalytic activity were then tested. Rh/?-Al2O3 was deactivated in all three experiments, likely due to significant carbon accumulation on/near the Rh metal. The activity of the pyrochlores in the presence of the contaminants was LSRZ>LRZ>LZ, which was directly related to carbon formation on the surface. Both LZ and LRZ were irreversibly poisoned by MN and DBT while the activity of the LSRZ is only kinetically inhibited by these contaminants. The resistance to deactivation by LSRZ is thought to be attributable to the oxygen-ion conductivity that results from Sr substitution into the pyrochlore structure. The presence of Rh, in both LRZ or LSRZ, resulted in a greater resistance to deactivation by sulfur and carbon accumulation on active sites than the supported Rh/?-Al2O3.